Symbiotic associations between hermit crabs and bryozoans from the Otago region, southeastern New Zealand

Abstract

Thirteen species of bryozoans (six cyclostomes and seven cheilostomes) occurring off the Otago Peninsula (southeastern New Zealand) form symbiotic, possibly mutualistic, associations with hermit crabs. For all but one of these bryozoan species, such an association has not been reported previously. At most only 3 of the 13 bryozoans are obligate symbionts of hermit crabs. Associations are apparently initiated when a bryozoan larva settles on a gastropod shell occupied by a juvenile hermit crab and develops to form a colony which encrusts the whole shell and then continues to grow out beyond the shell aperture in the form of a helicospiral tube. The tube-building bryozoan colony grows in step with the crab, and tube development appears to be controlled by crab morphology and activity. Of the six species of hermit crabs found occupying bryozoan tubes, four were regular tube occupants, although they could also be found in other shelter types. Hermit crabs occupying bryozoan tubes very rarely indulged in shell exchanges or shell fights. There appear to be no clear cut pairwise correlations between hermit crab species and tube-forming bryozoan species. The following bryozoan species are described for the first time: Heteropora parapelliculata, Borgiola otagoensis, Disporella gordoni, Osthimosia monilifera and O. socialis.